1. Field of the Invention
The present invention relates to a method for producing a resin article having an undercut shape, that is, a shape having a part which prevents removal of the molded article from a mold, by an injection blow molding method. Such a part is, for example, a part larger than an opening as shown in FIG. 5.
Resin articles having an undercut shape are used as shades of light fittings, containers, cases for industrial parts and parts of light electric appliances, and the like.
2. Description of the Related Art
In general, resin articles having an undercut shape are produced from thermoplastic resins by (1) injection molding, (2) press molding, (3) vacuum and pressure forming, and (4) injection blow molding. (1) The injection molding comprises the steps of providing male and female molds which leaves a space having the same shape as a resin article to be produced between them, supplying a synthetic resin in a molten state in the space, cooling and solidifying the resin, and then separating the molds to remove the resin molded article.
It is very difficult for the molds to have a structure from which the resin article is removed, when the outermost size of the resin article is larger than the opening of the article.
(2) The press molding comprises the steps of producing an intermediate resin article of a flat plate shape, placing the intermediate article which is still maintained at a high temperature on a female mold, and pressing the intermediate article between male and female molds to shape it in a desired shape.
However, it is impossible to press mold an article when its outermost size is larger than its opening. Furthermore, the press molding should be carried out using a line of large molding machines, since the intermediate article should be pressed while it is maintained at a high temperature.
(3) The vacuum or pressure forming comprises the steps of producing an intermediate resin article of a flat plate shape, placing the intermediate article on a female mold, clamping the periphery of the article, heating the clamped peripheral parts, and inflating the intermediate article to press it against the female mold by evacuating an air from the space between the female mold and intermediate article through an air vent hole provided in the female mold or by blowing a compressed air over the inner surface of the intermediate article.
In this molding method, shapes of projected parts are limited. Furthermore, articles which will be screwed in the opening cannot be produced because of the mold structure, and the clamped parts remaining at the periphery of the opening should be removed by post-processing. Furthermore, the molding cycle is prolonged and thus these methods are not suitable for mass production.
(4) The injection blow molding comprises the steps of producing an intermediate resin article having a shape similar to that of a final resin article by injection molding, removing a female mold while the intermediate article is maintained at a high temperature, inserting the intermediate article formed around a male mold in a female mold having the same shape as that of the final resin article, and inflating the intermediate article to press it against the female mold by blowing a compressed air through a hole provided in the male mold wall.
This method continuously carries out the injection molding and blow molding. Thus, the intermediate articles are less contaminated by handling such as transportation. The productivity of this method is high.
This method can produce a resin article having the outermost size which is larger than the opening to a certain extent, but the undercut shapes to be produced by this method are still limited.
The resin articles having the undercut shape which are produced by the above conventional methods have the increased tolerance of wall thickness as the outermost size becomes large in comparison with the opening. In particular, in the case of molded articles which have a small height and have been highly stretched such as shades of light fittings, tip ends of corners or corners of square shaped articles have thin walls, and thus the articles have the low strength for practical use. In addition, lamp images are seen through the thin wall parts in the case of light-diffusing type shades of light fittings.
Thus, all the conventional molding methods have the drawback that the undercut shapes to be produced are limited.